A printer, in particular an inkjet printer, generally includes one or more ink cartridges. Each ink cartridge has a printhead with ink nozzles. Ink droplets are expelled through the ink nozzles onto a print medium advanced through the printer. The ink cartridges are normally mounted in a carriage of the printer. The carriage is movable across a width of the print medium by sliding along a guiding rod. Images are formed on the print medium by controlling the movement of the carriage, and hence the ink cartridges, across the print medium and expelling ink droplets from the ink nozzles onto the print medium advanced through the printer accordingly.
The quality of the images formed depends on the trajectory of the ink droplets from the ink nozzles to the print medium. One of the factors affecting the trajectory of the ink droplets is the spacing between the printhead and the print medium. This spacing is commonly referred as the Pen-to-Paper Spacing (PPS).
For high quality images, the PPS should be minimized. This is because as the PPS increases, the trajectory of the ink droplets changes and becomes difficult to predict. Moreover, the ink droplets also start to spread out when the PPS is large, resulting in a “spray” effect. The “spray” effect is due to the forming of secondary ink droplets from the primary ink droplet. These secondary droplets introduce noise to the images form, and hence, reduce image quality.
Although the PPS should be minimized to obtain high quality images, it should not be too small such that it contacts the print medium. In other words, sufficient spacing between the printhead and the print medium should be ensured to prevent smearing of images on the print medium or damage to the printhead as a result of the print medium contacting the printhead. Contacting of the printhead by the print medium may be caused by the upward buckling of the print medium toward the ink nozzles due to the absorption of ink in the print medium. Another possibility of contacting the printhead by the print medium may be due to the use of a thick print medium. Therefore, in order to obtain the highest quality of images, an optimum PPS should be maintained.
Print medium includes paper, cardboard and Compact Disc (CD), all of which have different thicknesses. Therefore, the PPS in the printer changes when printing on different types of print medium with different thicknesses. Printers normally have mechanisms to provide different PPS settings for printing on different types of print medium.
The ink cartridge is normally mounted in the carriage in such a way that the printhead faces the print medium at a front portion of the carriage. The carriage is also normally mounted at its central portion on the guiding rod. For such a design arrangement, the PPS in the printer can be increased by pushing a rear portion of the carriage downwards. Since the carriage is mounted on the guiding rod at its central portion, the front portion of the carriage is tilted upwards. Therefore, the PPS in the printer is increased. However, when the front of the carriage is tilted upwards, an angle between the printhead and the print medium (theta X) increases. The increase of theta X leads to high dot placement error.
In order to maintain theta X at zero, i.e. to maintain the printhead to be parallel to the print medium, the carriage may be mounted on two guiding rods at both the rear portion and the front portion. In this case, the printhead of the ink cartridge is located at the central portion of the carriage. To increase the PPS, both the guiding rods are lifted. By lifting the two guiding rods by the same amount, the PPS can be increased with theta X maintained at zero. However, this dual guiding rod design results in the printer to be high in cost, and also requires user intervention for lifting the carriage.
It is desirable to have a low cost mechanism for providing different PPS settings for different print medium and also able to maintain zero theta X at the different PPS settings.